Noise effect on core bias power supply for Klystron in LCLS

Linac Coherent Light Source (LCLS) at SLAC is driven by eighty klystrons. Each klystron is powered by a modulator with up to 24 kV pulse at 120Hz repetitive rate. A pulse transformer in the klystron tank will transform the high voltage pulse to 350kV to drive the klystron. The modulator also includes a DC power supply as a core bias for the pulse transformer. The eighty core bias power supplies run mostly stably. Some noises are observed on the operating core bias power supply and cause instability. In this paper, the unstable symptoms will be presented and some analyses of the noises will be presented as well.